I wasn't originally going to elaborate on why Sarah felt the need to figure out the spell on her own, but then an onslaught of questions happened, and I figured "yeah, better elaborate".
And hey, let's elaborate even further via the magic of commentary! YAY!
1 - Spellbooks are for those who have awakened. They're basically just blank books if someone hasn't awakened, and a magic mark doesn't count. Note that Jerry gave Susan a spellbook after she awakened and didn't even ask if she already had one.
2 - Tedd has to see a spell being used to analyze it, and Sarah can't intentionally cast her spell without having some idea what it is. Tedd doesn't have any ability that will help Sarah figure out what the spell is before she's ever used it.
3 - Not mentioned in the comic is the loud magic-detecting wand, but at least one person asked about it. That wand could confirm that Sarah does, in fact, have a magic mark. Beyond that, it would be useless.
In short, Sarah wants to guess what the spell is so her first time using it doesn't involve accidentally turning a classmate into a frog or something, and there isn't really any way anyone can help her with it beyond more guessing and being a potential accidental target for whatever her spell might be.